CD8⁺ T cells are commonly divided into naïve CD44^loCD122^lo and “memory phenotype” CD44^hiCD122^hi cells. Here we show data suggesting that these two cell populations represent independent CD8⁺ T cell subsets. Whereas IL-15^−/− mice lack CD44^hiCD122^hi CD8⁺ T cells, mice deficient in the kinase ITK lack CD44^loCD122^lo cells among CD8⁺ T cells. The same defects were observed during thymus development. CD44^hiCD122^hi cells were found among double-positive thymocytes and increased in frequency during CD8 development in wild-type mice. At the mature stage, IL-15^−/− mice harbored virtually no CD44^hiCD122^hi CD8⁺ thymocytes. In contrast, ITK^−/− mice lacked CD44^loCD122^lo CD8⁺ cells at this stage. We generated mice with genetic deletions in both IL-15 and ITK and observed a severe reduction of all CD8⁺ T cells. The two CD44^loCD122^lo and CD44^hiCD122^hi CD8⁺ T cell subsets differed in the periphery in that natural killer (NK) receptor expression was found only on CD44^hiCD122^hi CD8⁺ T cells. This expression was paralleled by their ability to respond to both T cell receptor and NK receptor engagements. In contrast, CD44^loCD122^lo CD8⁺ T cells mounted stronger responses to T cell receptor stimulation but failed to recognize NK receptor ligands. Thus, whereas ITK-dependent CD44^loCD122^lo CD8⁺ T cells appear to represent conventional CD8⁺ T cells, IL-15-dependent CD44^hiCD122^hi CD8⁺ T cells may have functions in both adaptive and innate immunity.